Summary

Yellow fever is a viral hemorrhagic fever with a case fatality rate up to 50%. It is caused by yellow fever virus (YFV), a mosquito-borne flavivirus that is related to dengue and Zika viruses. Despite an effective vaccine (17D), the virus still causes major outbreaks, as occurred in Brazil between December 2016 and March 2018 where there were >2000 confirmed cases, including >500 deaths, as well as >4000 epizootics (yellow fever in nonhuman primates) (1). On page 894 of this issue, Faria et al. (2) provide a genetic investigation of the outbreak in Brazil from December 2016 to October 2017, demonstrating the origins and movement of YFV during the outbreak. They determined that the outbreak originated in northeastern Brazil and moved southward to areas where the virus had not been found previously. Surprisingly, YFV moved at a rate of 4.25 km/day, which probably explains the magnitude of the outbreak. Modeling infectious disease outbreaks with phylogeographic tools (based on the geographic distribution of viruses according to viral genome sequence) as well as phylodynamic tools (based on the interaction of epidemiologic, immunologic, and evolutionary factors in viral genetics) has played a critical role in understanding outbreaks and developing public health countermeasures.